Yosemite Valley is quite accessible to people with Disabilities
You can download their information here.

The trail to Yosemite Falls is easily done. It is paved and only a slight upgrade when walking. You can use a wheelchair on this path.

 The picture below is my youngest on the trail to
Bridal Veil Falls. She always likes to rest. The falls are
beautiful in the spring, as you can tell by the pictures.

The beautiful valley of Yosemite. The Merced River was so high and beautiful the day we went. At times it has flooded the campgrounds. In fact, they have removed many of the lower campgrounds, because of this.
I would not hesitate to take a wheelchair to Yosemite Valley. There are so many things you can do, right on the valley floor. Also the shuttles have wheelchair access, so you can park your car and be on your way.
Another amazing thing that is offered, is a free, lifetime pass, to all National Parks. At Yosemite, it is available at the visitor center and the guide says, at the gate. Lines can be long at the gate but you can also pay $10 and do it online. They do require some proof. In my case, I had the girls with me, so they didn't require anything else. I got mine so long ago, they are almost unreadable. You also get 50% off camping.
Look at the link, and you can get all the information you need, for this beautiful park.
Happy touring.